1. Field of the Invention
The present invention relates to ionizing radiation detectors. In particular, the present invention relates to germanium well detectors and methods for using such detectors with small, low-activity samples.
2. Discussion of Background
Nuclear material accountability is a concern in all nuclear materials production and handling processes, including the production of nuclear fuels, radiation sources, and radioactive tracers for medical imaging. Accountability can be improved by systematically tracking nuclear materials throughout the manufacturing process. Also, there is special concern about the possible loss or diversion of nuclear materials, notably materials useful in weapons production, during production or transport. Even trace quantities of materials deposited within processing equipment and pipelines can add up to significant losses over a period of time. This concern has led to a greater emphasis on nuclear materials accountability to provide assurance that the types and amounts of nuclear materials in a facility are known.
As an example of a particular process in which nuclear material accountability is used, the fabrication of uranium/aluminum fuel for some types of reactors involves melting and casting the uranium/aluminum alloy, followed by a series of machining, cutting, and extrusion steps. Scrap materials from the machining and cutting steps are melted down and blended with new materials in succeeding casting steps. A large fraction of the melt material in a typical process is recycled scrap. Variations in the uranium content of the starting material, variations in the amount of recycled scrap in the melt, and deposition of small amounts of material in the process equipment combine to introduce uncertainties in uranium accountability procedures. These uncertainties could be significantly reduced by measuring the uranium-235 (.sup.235 U) content of all production melts.
A number of techniques are available for analyzing the composition of materials by detecting ionizing radiation from the materials. For example, the composition of coal on a conveyor belt can be determined by detecting the transmission or scattering of X-rays or gamma rays therethrough (Watt, et al., U.S. Pat. No. 4,566,114). Other techniques for analyzing coal samples are based on detecting the transmission characteristics of beams of two different energies through the material (Watt, et al., U.S. Pat. No. 4,090,074; Wykes, et al., U.S. Pat. No. 4,359,639). The relative proportion of .sup.235 U in a mixture of .sup.235 U and .sup.238 U can be found by monitoring the alpha activity of the mixture with a solid state detector (Allenden, et al., U.S. Pat. No. 3,321,626). These techniques are not readily adaptable to the problem of measuring the .sup.235 U content of small or low-activity samples.
Semiconductor-based detectors are frequently used to detect ionizing radiation in the X-ray and gamma ranges. Such detectors typically have a P-I-N structure, that is, an intermediate zone of intrinsic or impurity-compensated semiconductor material (I) sandwiched between the p and n layers of a diode. When a reverse bias voltage is applied to the diode, the electric field across the diode sweeps charge carriers towards the p and n regions, creating the I region, or depletion zone.
The depletion zone is sensitive to low energy photons (X-rays and gamma rays). Photons interact with the material in the depletion zone primarily via the photoelectric effect, Compton scattering, and pair production. The photoelectric effect predominates at relatively low energies (up to about 100 keV); in most materials, Compton scattering is most important at energies above about 1 MeV. Pair production begins to predominate at energies of 5 to 10 MeV, depending on the composition of the absorber, but these energies are greater than the energies of decay gammas. Electrons and holes created by these mechanisms are swept by the electric field to the p and n regions. Each event produces a short (on the order of a .mu.sec), electrical current pulse with an amplitude proportional to the energy deposited in the detector by the incoming photon. The current pulse is converted to a voltage pulse by an instrument such as a charge-sensitive preamplifier. A pulse height analyzer records the number of pulses or counting rate, the amplitude of each pulse, the energy distribution, and so forth. The efficiency and resolution of the detector depend on the counting geometry, or relative configurations of the source and detector, as well as the volume of the depletion zone and the bias voltage, both of which should be as large as possible. These factors, together with the energy of the incident radiation, all affect the number of detectable events that appear in the pulse-height spectrum.
Semiconductor detectors provide good energy resolution (about 1.5 keV) compared to other well-known types of radiation detectors such as ionization chambers, proportional counters, scintillation detectors, and so forth. The efficiency of detector materials depends on their atomic number (Z). Thus, silicon (Z=14) is suitable for use in the relatively low energy X-ray range, while germanium (Z=32) is used in higher energy X-ray and gamma-ray ranges.
Germanium detectors are preferred to other types of semiconductor detectors because they have a low concentration of net residual active impurities (less than 5.times.10.sup.10 cm.sup.-3) and small statistical variations in the electrical signals resulting from incident X-ray and gamma radiation. Also, they are highly stable and need not be stored at cryogenic temperatures. However, germanium has a relatively small band gap (0.74 eV), so germanium-based detectors have high thermal noise levels and correspondingly high leakage currents at room temperature. Therefore, they must be operated at cryogenic temperatures to minimize leakage current-induced noise.
A number of hyperpure germanium detectors are available. A coaxial detector comprises a cylinder of germanium with an n-type contact of diffused lithium and a p-type contact of implanted boron. One of these contacts is on the outer surface of the cylinder and the other on the surface of a narrow axial well. Raudorf (U.S. Pat. No. 4,237,470) and Harchol (U.S. Pat. No. 4,056,726) describe typical coaxial detectors. When a reverse bias is applied between the contacts, ionization caused by radiation incident on the outer surface permits electrical current to flow in pulses. The sample to be measured is positioned on or near the detector. Since radiation is emitted uniformly in all directions, only a small fraction of the radiation emitted by the sample reaches the detector.
Well detectors have a larger axial well than coaxial detectors. The counting geometry, that is, the configuration of the sample and detector vis-a-vis each other, is improved over that obtainable with a coaxial director by placing the sample directly into the well because more of the radiation emitted by the sample reaches the detector material. Well detectors are thus ideal for low-activity samples or cases where the sample size is small. Hyperpure germanium well detectors are available from several sources, including Canberra Nuclear Products Group, EG&G Ortec, and Princeton Gamma Technologies.
The data obtained with coaxial detectors and well detectors must be calibrated and corrected for sample self-attenuation. Self-attenuation effects are due to absorption of some of the gamma rays emitted by a sample by the sample material itself. Because these gammas never exit the sample and never reach the detector, they do not contribute to the counting rate. The magnitude of this effect depends on the composition of the sample, the volume and depth of the sample, and so forth. Additional data obtained with a transmission source--a radiation source having a stable, uniform output in the energy range of interest--can be used to correct for self-attenuation effects. A shuttered transmission source is positioned so that the sample is between the transmission source and the detector. The counting rate is measured with the shutter open (sample and transmission source) and closed (sample only). These data are used to determine a correction factor for the sample self-attenuation. A suitable transmission source for measuring the .sup.235 U content of a sample is .sup.169 Yb, which has two peaks, at 177.2 keV and 197.9 keV, bracketing the .sup.235 U peak at 185.7 keV. However, .sup.169 Yb has a relatively short half-life (32 days), so .sup.169 Yb sources must be frequently replaced or reactivated.
A reference source adds a known, stable output to the recorded data. For example, to determine the .sup.235 U content of a sample, the reference source preferably has an energy peak near the .sup.235 U peak at 185.7 keV. Data from the known source are used in computing the .sup.235 U content of the sample from the 185.7 keV peak data. When the sample is between the reference and the detector, some of the reference photons are absorbed by the sample, leading to uncertainties in the counting rate. Ytterbium-169 is a suitable reference source for .sup.235 U measurements, since its two peaks provide a good correction factor for these uncertainties. However, as noted above, .sup.169 Yb sources are short-lived and therefore expensive to use.
Both coaxial detectors and well detectors can be used to determine the .sup.235 U content of uranium/aluminum samples. However, when coaxial detectors are used, the best results are obtained with relatively large samples. Smaller samples minimize personnel exposure to radiation, but have intrinsically lower counting rates and require longer measurement times. In addition, self-attenuation corrections must be determined for each sample due to unavoidable variations in the composition of successive production melts. There is a need for a method and apparatus for making fast, accurate measurements of the radioactive isotopic content of small, low-gamma-activity samples.